1. Field of the Invention
The present invention relates to a double-sided printing apparatus, and more particularly to a double-sided printing apparatus including double-sided stencil printing apparatus.
2. Description of the Related Art
Technologies relating to the present invention are disclosed in the following:
[Prior Art 1] JP Laid-open Patent Publication No. H6-48014
[Prior Art 2] JP Laid-open Patent Publication No. H6-71996
[Prior Art 3] JP Laid-open Patent Publication No. H7-81202
[Prior Art 4] JP Laid-open Patent Publication No. H8-118774
[Prior Art 5] JP Laid-open Patent Publication No. H9-95033
[Prior Art 6] JP Laid-open Patent Publication No. H10-129100
[Prior Art 7] JP Laid-open Patent Publication No. 2003-200645
[Prior Art 8] JP Laid-open Patent Publication No. 2003-266906
[Prior Art 9] JP Laid-open Patent Publication No. 2004-224479
A commonly known example of a printer is a thermal digital double-sided stencil printing apparatus (hereafter simply referred to as a stencil printing apparatus). This printing method uses a stencil plate master (hereafter simply referred to as a “master”), which has a laminated structure formed from a thermoplastic resin film applied to a porous support member. The thickness of the thermoplastic resin film is normally between 1 to 2 μm. The porous support member is made from Japanese paper fibers, or synthetic fibers, or a mixture of Japanese paper fibers and synthetic fibers. The thermoplastic film surface of the master is thermally stenciled to form the master by contacting heating elements of a thermal head operated in the main scanning direction of the thermal head. The master that has been stenciled (hereafter sometimes referred to as a stenciled master) is transported in the sub scanning direction, which is normal to the main scanning direction, by master transport means such as platen rollers or the like, and wound around a porous cylindrical shaped rotatable plate cylinder. The plate cylinder is formed by winding a plurality of layers of resin or metal mesh screens. Ink is supplied to the stenciled master on the plate cylinder from an ink supply member within the plate cylinder. Using pressure means such as a press roller, pressure cylinder, or intermediate pressure roller (hereafter generically referred to as “press roller”) the stenciled master on the plate cylinder is directly and continuously pressed against a sheet shaped recording medium such as for example print sheets (hereafter referred to simply as “sheets”). Printing is carried out by forcing ink through the perforations on the plate cylinder and the master, and transferring the ink to the sheets. Also, a stencil transfer printing apparatus is commonly known, in which ink forced from the perforations of the plate cylinder is temporarily transferred to a transfer cylinder having a rubber sheet, and then indirectly printed onto sheets (for example, see Prior Art 1).
Note that “plate cylinder” sometimes refers to a printing drum, or sometimes to the outer periphery of a printing drum. However, in this patent specification “plate cylinder” refers to the entire printing drum.
In recent years most stencil printing apparatus carry out double-sided printing on both the front and reverse sides of a sheet to reduce the consumption of sheets and storage space for documents, in addition to single-sided printing on one side of a sheet only. Conventionally the double-sided printing method and format uses the normal stencil printer apparatus that carries out single-sided printing as described above. Sheets stacked in the sheet supply unit are supplied to the printing unit, where printing is carried out on one side (the front side). The printed sheets are then discharged and stacked in the discharge tray. The sheets are then reversed, and again supplied to the printing unit, where printing is carried out on the remaining side (the reverse side), to obtain double-sided printing. In this double-sided printing method, the total printing time is very long because printing is carried out twice, and waiting time is necessary after completion of single-sided printing until the ink has dried on the front side, or, as it is referred to, until the front side has set. In addition the work of re-arranging the single-sided printed matter or re-setting the single-sided printed matter in the sheet supply unit was very labor intensive.
In order to improve this manual operation associated with the conventional double-sided printing method, there has been vigorous development of double-side printing apparatus that can automatically carry out double-sided printing, and several methods have been proposed for the format of the double-sided printing apparatus.
For example, in Japanese Patent Application Laid-open No. 2003-266906 (Prior Art 8 shown above), conventional double-sided printing apparatus is generally classified into six methods. In (1) the two drum in opposition one pass simultaneous double-sided printing method, two plate cylinders are provided in mutual opposition, and a sheet can be printed on both sides in one pass. In this method, the apparatus is large, and there is the restriction that when carrying out single-sided printing it is necessary to fit an unstenciled master to one plate cylinder to prevent transfer of ink from that cylinder. This results in wasteful consumption of masters, the work is troublesome, and other problem points (see for example, Prior Art 2).
The other remaining five types of double-sided printing method are: (2) the two pass double-sided printing method with stock re-supply after single-sided printing (see for example, Prior Art 3), (3) the single pass double-sided printing method with two drums in opposition and a transfer cylinder in between (see for example, Prior Art 4), (4) the double-sided printing method with a single drum sub-divided and simultaneous reversal (see for example, Prior Art 5), (5) the double-sided printing method with a single drum sub-divided printing and transfer drum (see for example, Prior Art 6).
Finally, although there are restrictions on sheet size and sheet type, (6) is a revolutionary single process double-sided printing apparatus that generally solves the problem points of (1) through (5) above, that is capable of single-sided printing without using masters unnecessarily, and is capable of providing high quality printed matter when double-sided printing. Further, the increase in installation space can be reduced. This adopts the double-sided format of (4) as the basic method (hereafter referred to as the “one drum one pressing means double-sided printing method” or the “one plate cylinder one pressing means double-sided printing method”). This is a new low cost double-sided printing apparatus that has been proposed to solve and provide measures against problem points such as soundness and reliability of sheet transport, and lack of adaptability to high speed printing (see for example, Prior Art 7 through 9).
This is a method of carrying out double-sided printing in which a single stenciled master wound around a single plate cylinder is divided into the master for printing the front side and for printing the reverse side, as shown in Prior Art 8. This format carries out double-sided printing by continuously pressing the unprinted side of sheets that have been printed on the front side (sheets that have been printed on one side) using one of the sub-divisions of the sub-divided master on the plate cylinder. This is accomplished by re-supplying sheets by clever use of sheet reversal and transport by the rotation of the single pressing means (in particular, a press roller having a diameter smaller than the external diameter of the plate cylinder).
In the double-sided printing apparatus disclosed in Prior Art 7 and elsewhere, for resist of reversed sheets that have been printed on one side and to correct skew, and so on, the front edge of a sheet is temporarily stopped by predetermined sheet re-supply stopping means (equivalent to the sheet re-supply position determination member in the prior art documents). Then the sheet is slightly moved and stopped by a sheet re-supply transport device (equivalent to the sheet re-supply transport member in the prior art documents) as sheet re-supply transport means. Then at a predetermined timing a sheet re-supply resist roller (equivalent to the sheet re-supply resist member in the prior art documents) as sheet re-supply resist means, provided in a stopper member positioning unit as sheet re-supply stopping means, operates and rotates. Then the sheet that is printed on the front side contacts the press roller, and is transported reversed to the printing unit by the rotation transport operation of the press roller, where double-sided printing is carried out.
The reversed sheet is temporarily stopped by the predetermined stopping means in order to carry out resist, correction of skew, and so on. Thereafter at a predetermined timing transport means transports the sheet to the printing unit again, where printing is carried out. However, after stopping the sheet is again suddenly transported at the linear speed of the drum, so there is variation in the position of the leading edge of the sheet when it arrives at the nip between the plate cylinder and the pressing means. This has the problem that there is variation in the resist after printing.